Generally, a voltage-source PWM method is used as an inverter control for driving a PM motor. In many cases, a vector control is applied to a current control of the motor, for the purpose of controlling a current waveform into a sine wave and thereby suppressing harmonics resulting in enabling a smooth control of the PM motor.
Moreover, in many cases, a sensorless method is used for detecting a magnetic-pole position of the PM motor. In this sensorless method, in general, a terminal voltage of the motor is taken into a CPU by analog-digital conversion and the like so that the magnetic-pole position is detected.
The motor current control and the sensorless method require a high arithmetic load (computational load) of the CPU, and hence, a computing period (cycle) needs to be sufficiently high-speed with respect to a rotational period of the motor. Moreover, since the computing period becomes higher in speed as a motor rotational speed becomes higher, it is difficult to adopt the above-mentioned method (the motor current control and the sensorless method) in a control for ultra-high-speed motor (having hundreds of thousands of rotations min−1).
A bottleneck of this problem is a computing-capacity limit of an arithmetic unit (computing unit) relative to the control method. It is said that about one hundred thousand rotations min−1 is a limit in consideration for computing capacity and analog-to-digital conversion time of an existing CPU.
As a countermeasure against this problem, Patent Literature 1 discloses a 120-degree-conduction pseudo-current-source inverter adopting the sensorless method. FIG. 8 is a configuration view showing a main circuit of the pseudo-current-source inverter whose input is a DC power source (this FIG. 8 is equivalent to a structure of Patent Literature 1 when regarding the DC power source as a diode bridge circuit in an input portion of FIG. 1 of Patent Literature 1). For comparison, FIG. 9 shows a main circuit configuration of a general voltage-source inverter. The pseudo-current-source inverter (FIG. 8) disclosed in Patent Literature 1 includes an additional voltage-drop circuit 2 as compared with the general voltage-source inverter shown in FIG. 9. The voltage-drop circuit 2 is constituted by a transistor FET 1, diodes D1 and D2, and a reactor L1.
In a control method for the pseudo-current-source inverter in Patent Literature 1, a 120-degree-conduction rectangular wave is given as the motor current, and then, an influent peak-current is controlled by the voltage-drop circuit 2. Because a controlled object is DC current that is flowing in the reactor L1, the control period (cycle) is not restricted to a high frequency of ultra-high-speed motor. By virtue of such a control method, the arithmetic load of the CPU which causes the above-mentioned problem in the case of ultra-high-speed motor can be reduced to enable the control for the ultra-high-speed motor.
However, although the pseudo-current-source inverter shown in FIG. 8 realizes the control for ultra-high-speed motor, the pseudo-current-source inverter shown in FIG. 8 needs the additional devices (transistor FET1, diodes D1 and D2, reactor L1) constituting the voltage-drop circuit 2. Further, a wire circuit for connecting these additional devices is complicated. As a result, the pseudo-current-source inverter has a disadvantageous structure as a whole of motor drive system, from viewpoints of size, cost and loss.
Recently, an inverter has been strongly required to attain cost reduction, downsizing and loss reduction. Particularly, in a case that the inverter is used in a very-limited installation space such as vehicle installation, the pseudo-current-source inverter shown in FIG. 8 renders prominent the disadvantage of large size so that it is difficult to install the inverter.
It is therefore an object of the present invention to provide an inverter control system devised to realize low arithmetic load and reduction in size, cost and loss.